Analyte Stability during Sample Storage

The analyte stability during storage and processing of samples or in standard solutions and extracts is not part of method validation in Germany. Therefore, insufficient stability will not be routinely detected and even then more or less only by chance. Also, separate tests for analyte homogeneity and extraction efficiency were not performed. 

ED is sometimes particularly susceptible to increases in background noise caused by metal ions present in the eluent, introduced from the samples, eluent or from components of the HPLC system. Some analysts add EDTA to their eluents, typically at concentrations of 30-150 mg L (0.1-0.5 mmol L ), with the aim of chelating iron and possibly other metal ions, EDTA may also enhance the stability of analytes, such as catecholamines, during sample storage and analysis. 

Stability testing is performed to ensure that the analyte concentration does not change during sample storage, sample preparation, and sample analysis. The freeze and thaw stability of analyte in matrix as well as the short-term stability of analyte in matrix at room temperature should be examined. The long-term stability of analyte in matrix stored in freezer should also be tested, as should the stability of stock solution and working solutions ofthe analyte and internal standard. The stability of the analyte (and IS) is satisfactory when the determined concentration is within the limits of accuracy. 

L.-P. Yu and X.-P. Yan. Factors affecting the stability of inorganic and methylmercury during sample storage. Trends in Analytical Chemistry 22 245-253, 2003. 

Field fortifications have also been used to measure the storage stability of the analyte in/on exposure matrices during freezer storage prior to analysis. Although use of field fortification samples for freezer storage stability is not the original purpose intended for field fortification samples, this has become an acceptable practice among scientists who work in this scientific discipline. 

Freeze and Thaw Stability. Analyte stability should be determined after three freeze-thaw cycles. At least three aliquots at each of the low and high concentrations should be stored at the intended storage temperature for 24 h and thawed unassisted at room temperature. When thawed completely, the samples should be refrozen for 12 to 24 h under the same conditions. The freeze-thaw cycle should be repeated twice more, then analyzed on the third cycle. If an analyte is unstable at the intended storage temperature, the stability sample should be frozen at —70°C during the three freeze-thaw cycles. 

A number of studies have highlighted the degradation of residues during frozen storage, including P-lactam antibiotics in milk ampicillin in pig muscle chlortetracycline in incurred pig muscle, liver, and kidney " sulfamethazine in incurred pig muscle and bovine milk and gentamicin residues in egg. The EU validation criteria require that stability be determined for the analytes in matrix and in solution at various stages of the sample preparation process. Preferably, incurred tissue should be used otherwise... 

Equally important during the sampling, storage, and sample preparation stages of an analysis is the need to avoid the introduction of contaminants from the environment, storage conditions, or sample container. The adsorption of the analyte onto the surface of containers can be confused with instability and complicates the sample stability and preservation problem. 

The stability of the analyte(s) in matrix during extended freezer storage should be determined by analyzing stored stability QC samples at appropriately selected time intervals and at temperatures that reflect the intended storage conditions and anticipated storage periods for study samples. For many applications, the compound is determined to be stable as long as the calculated concentration of the analyte is within 15 % of the nominal concentration, or the concentration that was established for the same batch of QCs when analyzed immediately after preparation (time zero). Freshly prepared QC samples, or QC samples prepared and stored within an established period of stability, should be used as analytical QCs in the same set that the stability QCs are analyzed to confirm run acceptance. If the analytical QC samples do not meet assay acceptance criteria, the run should be rejected and the stability interval should be repeated. Incurred samples or sample pools may also be analyzed for assessment of stability in a similar manner. 

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